The present invention relates generally to a nonvolatile storage devices and methods of operation such devices, and more particularly to a magneto-resistive random access memory (MRAM) that may include a magnetic resistance element as a storage element, and an operating method for such an MRAM.
A magneto-resistive random access memory (MRAM) can typically utilize a magnetic-resistance (MR) element as a data storage element. Data can be written to a MR element by establishing a magnetic state for such an element. A change in the magnetic state of an MR element can correspond to a change in resistance for the MR element. Thus, data may be read from a MR element by detecting a voltage established by the resistance of a MR element.
Typically, the magnetic state of a MR element can be set or changed by establishing a magnetic field according to a current flowing within the vicinity of the MR element. In this way, data can be written to a MR element.
Read and write operations for a conventional nonvolatile storage device, such as a conventional MRAM, will now be described with reference to FIG. 15. In FIG. 15, a nonvolatile storage device 1500 may include a number of memory cells, one of which is shown as 1502. Each memory cell (e.g., 1502) of a nonvolatile storage device 1500 may include a metal-oxide-semiconductor (MOS) transistor (one of which is shown as 1504) and a magnetic resistance element (one of which is shown as 1506). A magnetic resistance element (e.g., 1506) can be connected between a MOS transistor (e.g., 1504) and a write word line WW. A magneto resistance element may be referred to herein in some cases as TMR. A nonvolatile storage device 1500 may also include a bit line BIT LINE that may intersect a write word line WW and a read word line READ WORD LINE in the vicinity of a TMR. A read word line READ WORD LINE can be parallel to a write word line WW.
In a conventional write operation, one of a number of write word lines can be selected. When a write word line is selected, a write current, generated by a current generating source, can flow through the write word line WW. Further, a bit line write current can flow through a bit line B to further effect data writing. A magnetic state of a magnetic resistance element 1506 can be set according to a synthetic magnetic field formed by currents supplied to a write word line WW and a bit line B. A magnetic state of a magnetic resistance element 1506 can be altered according to a direction of current flow within a bit line B. Thus, a current flow in one direction within bit line B can establish one particular data value, while a current flow in an opposite direction within bit line B can establish another data value. Current flow direction in a bit line can be varied by a bit line voltage.
In a conventional read operation, a read voltage (a power source voltage) is applied to a read word line WR in order to read data values. In addition, a voltage of about 0.5 volts (V) can be applied to a write word line. A magnetic state of a magnetic resistance element 1504 can determine the resistance of a magnetic resistance element for memory cell selected by a read word line WR. Thus, when the above voltages are established, the resistance of the selected magnetic resistance element can establish a bit line voltage. This bit line voltage can be detected to thereby determine a data value stored within a selected memory cell.
As described above, in a conventional nonvolatile storage device using a magnetic resistance element, data may be written by current flowing in a word line and bit line. Conventionally, a current flow direction in a word line may be the same regardless of a data value being written. In contrast, a bit line current flow direction will vary according to the data value written. Accordingly, conventional write operations can include consistent generation of a word line current that always flows in the same direction.
It is also noted that in a conventional nonvolatile storage device data values may be biased (the occurrence of one data value may be more likely than another). In such a case, a bit line current direction may be likewise biased to flow in one direction.
A drawback to the above conventional write current arrangements can arise from electromigration. When current flows in one direction, or is biased to flow in one direction, within a conductor, electromigration may cause discontinuities to occur in the conductor. Consequently, in a conventional nonvolatile storage device, bit lines and/or word lines can suffer from electromigration induced discontinuities, resulting in undesirably high resistance or even opens in such bit lines and/or word lines.
In other words, when current flows in a wiring, an opening can develop in such a wiring due to electromigration. In cases where current always flows in one direction, or is biased to flow in one direction, electromigration may progress at faster rates than other arrangements. Still further, conventional nonvolatile storage devices may be more susceptible to such adverse effects, as a relatively large current may be required to induce the necessary magnetic field for establishing the magnetic state of a magnetic material (like that included in a magnetic resistive element). That is, larger currents can result in greater electromigration effects.
In light of the above, it would be desirable to provide a nonvolatile storage device that may not be as susceptible to the adverse effects of electromigration like that described above. In particular, it would be desirable to arrive at a nonvolatile storage device having word lines and bit lines that may be prevented from forming openings that could otherwise occur in a conventional nonvolatile storage device. It would also be desirable to arrive at a method of operating a nonvolatile storage device that could include the above-mentioned benefits.
The present invention may include a nonvolatile storage device that includes a number of word lines, a number of bit lines, and a number of magnetic resistance elements provided at intersections of the word lines and bit lines. In addition, a word line control circuit can select one of the word lines in a write operation and supply a bidirectional word line write current to the selected word line. A bit line control circuit can select one of the bit lines in a write operation and supply a bit line write current to a selected bit line.
According to one aspect of the embodiments, in the write operation, a word line control circuit can provide a word line write current in one direction and then in a reverse direction.
According to another aspect of the embodiments, an amount of time a word line write current flows in the one direction can be essentially equal to an amount of time a word line write current flows in a reverse direction.
According to another aspect of the embodiments, a bit line control circuit can provide a bit line write current in one direction and then in a reverse direction.
According to another aspect of the embodiments, a bit line control circuit may include a word linexe2x80x94bit line connecting circuit that can connect a bit line to a word line in a write operation.
The present invention may also include a nonvolatile storage device having a number of word lines, a number of bit lines, and magnetic resistance elements provided at intersections of the word lines and bit lines. A word line control circuit can select one of the word lines and supply a word line write current to the selected word line in a write operation. A word line control circuit may reverse a word line write current based on previous word line write current directions. In addition, a bit line control circuit can select one of the bit lines in the write operation and supply a bit line write current to the selected bit line.
According to one aspect of the embodiments, a word line control circuit can include a counter that counts the number of times that a write current of one direction is supplied to a word line. When the count reaches a predetermined value, the word line control circuit can reset the counter count and reverse a word line write current direction.
According to another aspect of the embodiments, a word line control circuit can include a counter that adds or subtracts a count value according to a direction of a word line write current. When the absolute value of the count reaches a predetermined value, a word line write current can be provided for a predetermined number of times for a predetermined duration to bring a count value to about zero.
According to another aspect of the embodiments, a word line control circuit can store a direction of a word line write current supplied to a word line. A word line control circuit may then reverse a word line write current direction in a next write operation.
According to another aspect of the embodiments, a nonvolatile memory device may also include a switching device corresponding to each word line. A switching device can selectively connect a word line to a predetermined supply potential.
The present invention may also include a method of operating a nonvolatile storage device. Such a method may include the steps of supplying a bidirectional word line write current to a word line selected by a word line control circuit when writing data to a magnetic resistance element formed at an intersection of the word line and a bit line. The method may also include supplying a bit line write current to the bit line, the bit line being selected by a bit line control circuit.
According to one aspect of the embodiments, supplying a bit line write current includes supplying a bit line with a bit line write current in one direction and then supplying a bit line write current in the reverse direction in a same write operation.
According to another aspect of the embodiments, supplying a bidirectional word line write current can include supplying a word line with a word line write current in one direction and then supplying a word line write current in a reverse direction in the same write operation.
According to another aspect of the embodiments, supplying a bidirectional word line write current can include supplying a word line with a word line write current in one direction prior to supplying a bit line write current.
According to another aspect of the embodiments, supplying a bidirectional word line write current includes coupling a word line to a bit line to supply a bit line write current.
The present invention may also include a method of operating a nonvolatile storage device that includes switching a direction of a word line write current in a write operation based on at least one previous word line write current direction when writing data to a magnetic resistance element formed at an intersection of the word line and a bit line. The method may also include supplying a bit line write current to a bit line, the bit line being selected by a bit line control circuit.
According to one aspect of the embodiments, switching a direction of a word line write current can include counting a number of times that a word line write current flows in one direction, and when the count reaches a predetermined value, resetting a counter count and switching a direction of the word line write current.
According to another aspect of the embodiments, a step of switching a direction of the word line write current can include storing a direction of a word line write current supplied to a word line and reversing a word line write current direction in a next write operation.
According to another aspect of the embodiments, supplying a bit line write current to a bit line includes supplying a bit line write current in one direction to a first bit line of a bit line pair and providing a bit line write current in an opposite direction to a second bit line of a bit line pair.
According to another aspect of the embodiments, a method may include, in a read operation, coupling a bit line to one terminal of a sense amplifying circuit and then writing a predetermined data value with a bidirectional word line current to the bit line, and coupling the bit line to another terminal of the sense amplifying circuit.